Heretofore, methods have been proposed whereby the amount of transmitted information is increased by transmitting different modulated signals from a plurality of antennas, and separating and modulating modulated signals transmitted simultaneously from individual antennas on the receiving side, as with the transmission method known as MIMO (Multi Input Multi Output) described in “Proposal for an SDM-COFDM System for Wideband Mobile Communications Achieving 100 Mbit/s by Means of MIMO Channels” (IEICE Technical Report RCS-2001-135, October 2001), for example.
This method will be explained briefly. A case will be considered in which, as shown in FIG.1, a modulated signal A and modulated signal B are simultaneously transmitted from two antennas T1 and T2 respectively, and modulated signals A and B are received by two antennas R1 and R2. In this case, it is necessary for four channel fluctuations h11(t), h12(t), h21(t), and h22(t) to be estimated on the receiving side.
Therefore, pilot symbols (radio wave propagation environment estimation symbols) 01, 02, 03, and 04 for estimating channel fluctuations h11(t), h12(t), h21(t) and h22(t) are placed in modulated signals A and B as shown in FIG. 2. Here, a C known signal is sent with pilot symbols 01 and 03, a-C* known signal with pilot symbol 02, and a C* known signal with pilot symbol 04. An asterisk (*) indicates a conjugate complex number. Apart from pilot symbols 01, 02, 03, and 04, data symbols are also placed in both modulated signal A and modulated signal B.
With a conventional radio transmitting apparatus that simultaneously transmits different modulated signals from a plurality of antennas in this way, embedding pilot symbols in the modulated signals transmitted from the antennas enables modulated signals multiplexed together on the propagation path to be satisfactorily separated and demodulated on the receiving side.
Heretofore, also, a system has been proposed whereby, taking the above-described technology as a basis, the number of antennas that transmit modulated signals is changed according to the radio wave propagation environment and so forth. With this system, when four antennas are provided on the transmitting side, for example, four different modulated signals are simultaneously transmitted using all four antennas when the radio wave propagation environment is good, whereas two different modulated signals are simultaneously transmitted using only two antennas when the radio wave propagation environment is poor.
However, with a system in which the number of modulated signals transmitted simultaneously is changed according to the radio wave propagation environment and so forth, the received signal level also changes in accordance with a change of the number of transmitting antennas (that is, the number of transmit modulated signals), and therefore the quantization error of an analog/digital converter in the receiving apparatus may become large. As this quantization error greatly affects the channel estimation precision and information data error rate, the modulated signal reception quality falls.